The adsorbents of this invention are useful in the adsorption and separation of gases. Preferably, the adsorbent compositions are used in processes for separating N2 from mixtures containing N2 and other gases by contacting the mixture with an adsorbent composition which selectively adsorbs the N2 with one or more of the less strongly adsorbable components recovered as product.
Of particular interest is the use of these adsorbents in non-cryogenic gas separation processes. For example, the separation of nitrogen from gas mixtures is the basis for several industrial adsorption processes, including the production of oxygen from air. In the cyclic production of oxygen from air, air is passed through an adsorbent bed having a preference for the adsorption of nitrogen molecules and leaving oxygen and argon (the less strongly adsorbable components) to be produced. The adsorbed nitrogen is then desorbed through a purging step, normally through a change in pressure, including vacuum, and/or through temperature changes to regenerate the adsorbent and the cycle is repeated. Such processes include pressure swing adsorption (PSA), temperature swing adsorption (TSA), vacuum swing adsorption (VSA) and vacuum pressure swing adsorption (VPSA) processes and such processes are commonly used in commercial air separation operations as well as in other industrial processes.
Clearly the particular adsorbent used in these processes is an important factor in achieving an efficient, effective and competitive process. The performance of the adsorbent is dependent on several factors, including the adsorption capacity for the N2, the selectivity between gases, which will impact the production yield, the adsorption kinetics, which will enable the adsorption cycle times to be optimized to improve the productivity of the process. The crush strength/attrition rate of the agglomerated particles is also very important particularly with respect to achieving a satisfactory adsorbent life in the adsorption process and system. Many of these factors are directly dependent on the particle pore structure and overall pore architecture.
The present invention is directed to novel adsorbent compositions, comprised of agglomerated adsorbent particles composed of at least one active component and a silicone-derived binding agent. The adsorbents produced therefrom show a surprising increase in adsorption capacity versus state of the art clay compositions. Additionally, the adsorbents are engineered during the manufacturing process to enhance their adsorption rate (kinetic) properties through improved composition (i.e. very high active phase concentration) and pore-structure architecture. Such adsorbents have high crush strength values and higher adsorption rate properties and are especially enabling for PSA/TSA/VSA/VPSA process intensification, a term commonly used to describe fast cycles with high rate adsorbents. When effectively used in these adsorption processes, such adsorbents lead to lower capital costs, reduced power consumption and/or increased product recovery.
Conventional agglomerated adsorbents used for such processes are composed of zeolite powders (crystallite particles), including ion exchanged zeolite powders depending on the process and binding agent. The binding agent is intended to ensure the cohesion of the agglomerated particles which are generally in the form of beads, pellets, and extrudates. Binding agents generally have no adsorbing property and their only function is to give the agglomerated particles sufficient mechanical strength to withstand the rigors of deployment in packed bed adsorption systems and the vibrations and stresses to which they are subjected to during the particular adsorption process, such as pressurization and depressurization. The particular binding agent and its concentration impact the final pore structure of the agglomerated particles thereby affecting the adsorbent's properties. It is known that the binding agent concentration should be as low as possible to reduce mass transfer resistances that can be negatively impacted from excess binder being present in the pores. Certain binding agents, temporary binders and other processing aids can also fill or otherwise partially plug the particle pores while other binding agents can have an adverse effect on the final pore structure depending on the particular binding agents' carrier solvents.
One of the most common methods to obtain agglomerated adsorbent particles with low binder concentrations, improved pore architectures and low mass transfer resistances is to use the caustic digestion method to prepare binderless adsorbents. Binderless adsorbents represent one approach to obtain a low binder content, but at the expense of additional manufacturing steps and higher costs. The conventional approach for caustic digestion is to employ clay binding agents that can be converted to active adsorbent material via the caustic treatment. Several prior disclosures have claimed novel pore structures and demonstrated various levels of improvement to the adsorption rate properties from the use of these binderless adsorbents.
For example, U.S. Pat. No. 6,425,940 B1 describes a high rate adsorbent made substantially binderless and having a median pore diameter >0.1 μm and in some cases a bimodal pore distribution having larger, 2-10 micron, pores engineered by using combustible fibers such as nylon, rayon and sisal, added during the forming process. In U.S. Pat. No. 6,652,626 B1, a process for producing agglomerated bodies of zeolite X is described wherein a binder containing at least 80% of a clay convertible to zeolite is contacted after calcination with a caustic solution to obtain an agglomerated zeolite material composed of at least 95% of an Li exchange zeolite X, having an Si/Al=1. The products are reported to have N2 capacities at 1 bar, 25° C. of 26 ml/g which corresponds to less than 26 ml/g at 1 atm and 27° C. No pore structure or diffusivity information is disclosed. In U.S. Patent Application Publication No. 2011/104494, a zeolite based adsorbent granulate is disclosed, comprising a zeolite of the Faujasite structure and having a molar SiO2/Al2O3 ratio ≧2.1-2.5. The adsorbent granulate has a mean transport pore diameter of >300 nm and a mesopore fraction of <10% and preferably <5%. The adsorbent granulate is prepared by mixing an X-type zeolite with a thermally treated kaoline clay in the presence of sodium silicate, sodium aluminate and sodium hydroxide.
A significant drawback to the manufacture of these binderless adsorbents is their high manufacturing cost due to additional processing steps, reagents and time required for the binder conversion. Another disadvantage of making binderless adsorbents stems from the need to handle, store and dispose of large quantities of the highly caustic solutions required in the adsorbent manufacturing process. This adds costs and environmental concerns to the process.
Another class of prior adsorbents teaches novel pore architectures through the use of novel binding agents or traditional binding agents with improved agglomeration processing. U.S. Pat. No. 6,171,370 B1 discloses an adsorbent showing utility in a PSA process which is characterized by having macropores with average diameter greater than the mean free path of an adsorbable component, when desorbing said component, and wherein at least 70% of the macropore volume is occupied by macropores having a diameter equal to or greater than the mean free path of the adsorbable component. The use of clay binders including attapulgite and sepiolite in concentrations of 5-30 wt % is described. U.S. Pat. No. 8,123,835 B2 describes the use of colloidal silica binders to produce superior adsorbents for gas separation applications including air separation. This teaching uses colloidal silica binding agents yielding macropores substantially free of binding agent. The adsorbents are characterized by an adsorption rate, expressed in the form of size compensated relative rate/porosity, of at least 4.0 mmol mm2/gs. The binder content is less than or equal to 15 wt % and the mean crush strength is greater than or equal to 0.9 lbF measured on particles having a mean size of 1.0 mm.
Other teachings use silicones as the binder precursor in various catalysts and related shaped bodies, such as honeycomb catalyst structures. For example, U.S. Pat. No. 7,582,583 B2 teaches shaped bodies, such as honeycomb structures, containing microporous material and one silicon-containing binder used for the production of Triethylenediamine (TEDA). The catalyst is formed by mixing the microporous material, the binder, a make-up aid and the solvent; forming, drying and calcining the structure. The make-up aid is cellulose or cellulose derivative, and the solvent can be selected from a list of various organic solvents. U.S. Pat. No. 5,633,217 teaches a method of making a catalyst, catalyst support or adsorber body by forming a mixture of ceramic and/or molecular sieves, silicone resin, a dibasic ester solvent, organic binder, and water. The mixture is shaped into a green body, dried and heated. U.S. Pat. No. 6,458,187 teaches a shaped zeolite-containing body prepared from a particular class of siloxane-based binders in combination with zeolite, plasticizing agent, and methylcellulose. The body is formed by mixing the components and calcinined at temperatures below 300° F. so as not to volatilize the methyl cellulose or other volatiles.
According to this invention, adsorbents for gas separation processes are provided which are made from free-flowing agglomerated particles. These adsorbents have high N2 adsorption rates, high N2 adsorption capacities, high crush strengths and attrition resistance, and are bound with low concentrations of total binding agents using less costly and traditional manufacturing processes. Further, the adsorbent compositions are characterized by a N2 adsorption capacity at 27° C. and 1 atm which is greater than an equivalent composition containing all clay binding agents.